This invention relates generally to the field of manufacturing, and more particularly, to the use of a fixture to secure a part in a machine tool during a manufacturing operation, and specifically to the manufacturing of precision parts with non-precision fixtures.
Machine tools are capable of very precise movements relative to the working surface of the tool. Typical manufacturing operations performed in this manner may include drilling, turning, milling, routing, welding, etc. The accuracy of such a manufacturing operation is limited not by the ability of the machine tool to perform a desired movement, but rather, by the ability of the operator to position the part accurately within the coordinate system of the machine tool. Highly skilled operators using precision measuring instruments are capable of performing machine/part setups for very precise operations. However, the cost of such precision setups is prohibitive for most applications involving high volume operations.
It is known to utilize fixtures to secure a part relative to the working surface of a machine tool during a manufacturing operation. The precision fixture is a mass production solution to the cost of precision setup. Rather than having a skilled machinist precisely position each part to be machined, the use of a precision fixture allows a machinist of lesser ability to rapidly and precisely place a part in a machine tool. In its simplest application, a skilled machinist would set up the machine and fixture, and a less skilled individual would then load the parts and operate the tool to perform the manufacturing operation. Conventional wisdom is that precise parts require very precise fixtures and that fixtures must be made to tolerances that are significantly smaller than their respective part tolerances. Standard practice for inspection or measurement operations is that the gauge tolerance should be only ten percent of the part tolerance. To a large extent, this standard is also applied to fixtures.
There are four problem areas commonly associated with precision fixtures. These problems are cost, availability, accuracy, and error documentation. Furthermore, these problems tend to be closely interrelated. The most basic of these problems is cost. In addition to the initial cost of manufacture, there are costs of validation, maintenance, and rework. Contact points on fixtures are subject to wear, and fixture shapes must be verified periodically, with occasional rework being necessary to return them to their specified dimensions. There is also the cost of setting up an incorrectly shaped fixture or adjusting the setup to compensate for part-to-part or lot-to-lot differences.
Availability is related to cost, since all too often, extra fixtures are held in inventory in case they are needed. The manufacture of a precision fixture can be a very time-consuming operation, and in most applications, the demands of production require that one or more spare fixtures be available at all times. In the aggregate, the cost of this inventory may be significant.
The problem of accuracy can be exacerbated in some applications, for example laser drilling. A small deviation in the shape of a fixture may translate into a very large error in the location of a machined feature. In a laser drilling application, for example, the holes in the surface of a part may be formed at a large angle relative to the surface normal at the drilling point. At angles such as 70 or 80 degrees from normal, even a small error in the location of the surface will result in a large error in the location of the hole.
The documentation problem is generated by the common practice of making manual adjustments to numerically controlled (NC) tool paths or the use of mechanical shims to adjust the location of a part within a fixture. Such practices may remain undocumented, and when the process is moved to a new location or discontinued for a period of time, such undocumented adjustments may be lost, resulting in cost and quality problems when the process is later reinitiated. In cases where fixtures are shimmed or NC programs are altered, the true as-manufactured product definition may be impossible to establish.
Thus, there is a particular need to develop a manufacturing process capable of producing precision parts without the use of precision fixtures. Accordingly, a method of performing a manufacturing operation on a part is described herein, the method including the steps of: creating a numeric model of a part; creating a numeric model of a fixture design having a plurality of contact faces for supporting the part in a design position relative to a machine tool coordinate system; creating a tool path program for controlling the operation of the machine tool in the performance of a manufacturing operation on the part in the design position; manufacturing a fixture based upon the fixture design; installing the fixture in the machine tool; measuring the location of the plurality of contact faces of the fixture; creating a numeric model of the location of the plurality of contact faces of the fixture relative to the machine tool coordinate system; nesting the model of the part into the model of the location of the plurality of contact faces; and, recording a transformation matrix describing the movement of a coordinate system of the part as it is moved during the step of nesting. The method further includes the steps of: using the transformation matrix to transform the tool path program to an as-fixtured tool path program; and using the as-fixtured tool path program to perform a manufacturing operation on a part disposed in the fixture.